1. Field of the Invention
This invention relates to gypsum models, original molds and forming molds, particularly gypsum molds for white wares or so-called plaster molds, which have an improved higher strength by dispersing a reinforcing material in a gypsum structure. It also relates to gypsum powder materials for such models and molds and a method for producing the models and molds and the powder materials.
2. Description of the Prior Art
Various method of increasing the strength of gypsum models, original molds and forming molds for general forming purposes (hereinafter, these molds are referred to simply as gypsum molds) have been known. One such method comprises mixing a major part of .beta.-hemihydrate gypsum with a minor part of .alpha.-hemihydrate gypsum so as to reduce a part of the water to be admixed. It will be noted here that hemihydrate gypsum may also be called plaster of Paris. Another method is known in which gypsum is admixed with cements or resins.
In a further method, gypsum is admixed with natural fibers such as hemp, or glass fibers.
However, the first-mentioned method in which .alpha.-hemihydrate gypsum is admixed with .beta.-hemihydrate gypsum so as to reduce a part of the water to be admixed is disadvantageous in that although the strength itself is slightly improved, the water absorbability which is essential for forming white wares is lowered. Good water absorbability of gypsum molds particularly at the time of slip casting is one of the most important factors required for gypsum forming mold. If the water absorbability of a gypsum mold is poor, a forming time for one product is prolonged, thus lowering the forming productivity. Especially, when the water absorbability of the slip casting molds at the time of slip casting is poor, the resulting green products are ill-shaped. Thus, the lowering of the water absorbability in a gypsum forming mold is a vital drawback.
In the case where gypsum is admixed with cements or resins, the strength of the molds or models may be improved with an attendant disadvantage that the water absorbability is lowered and their other properties change adversely. In the case of mixing gypsum with natural fibers such as hemp, natural fibers have so small a tensile strength as compared with synthetic fibers that an increase in strength cannot be attained unless large amounts of the fibers are mixed with gypsum. The use of natural fibers in an increased amount accrues to a lowering of the water absorbability of the resulting gypsum mold. In addition, single natural fibers are so thick that the fibers incorporated in the surface portion of the gypsum mold are liable to be exposed at their end portion at the forming faces. The exposed fiber ends at the forming faces may damage the surfaces of products being formed such as green ware and the portions where the fiber ends are exposed will cause a loss in water absorbability of the mold. Thus, the surface properties of the portions become uneven. In the case of a forming mold, forming failure tends to occur because of this uneven surface.
The method of mixing glass fibers with gypsum will slightly improve the strength of the gypsum mold. However, when the ends of the glass fibers incorporated in the surface portions are exposed on forming faces, because of the rigidity of glass fibers, the surface of the forming products such as green ware may be damaged.